This, together with a new try_config directive, will allow checking the
generic config and make it more flexible.

To reduce the number of packages that need to be listed in
GLUON_SITE_PACKAGES, this adds a new variable GLUON_FEATURES. Sets of
packages are enabled automatically based on the combination of listed
feature flags.

Site-specified package feeds can provide their own feature flag
definitions.

Fixes #1126

Don't stop after the first error, as later error messages may make the
cause of the error much clearer in some cases.

The current autoupdater will only respect the last line for a given model
name, so we can add SHA256 checksums as long as they occur before the
corresponding SHA512 line.

Setting DEVICES will also suppress cleaning the package directory,
improving incremental build speed.

The default abbreviation length depends on factors like user configuration,
git version and number of repository objects. Use unabbreviates IDs to make
patch generation more reproducible.

need_one_of(varname, array, required) checks weather the value of the specified variable is part of given array.
need_array_of(varname, array, required) is similar to need_one_of() but assume that varname points to an array.

Switching branches and applying patches in the build repos will
unnecessarily touch many files, causing rebuilds of packages that didn't
really change; furthermore, it is filling the reflog with many entries.

Don't ever switch to base branch in the build repos and apply patches in
a temporary clone to avoid these issues.

In addition, GPG signing is generally disabled in the build repos to
override potential global configuration (as signing doesn't make sense and
will slow down rebases).

git 2.9 enables rename detection by default, changing the content of some
of our patch files. Explicitly disable it.

Suggested-by: Steffen Möller <steffen_moeller@gmx.de>

Possibility to work with git submodules

This option will make the generated commit IDs deterministic, greatly
reducing the number of repository objects created when calling `make
update` repeatedly.

This will allow us to use its content from other languages than Lua as
well.

Using a loop makes the script a bit nicer, and it works around a git bug
corrupting patch dates when multiple patches are applied at once.

This fixes `make update` not working correctly when GLUON_SITEDIR is set
manually.

check_site.sh and site.sh were setting GLUON_SITE_CONFIG on their own
even though the environment variable was already exported.

If a given commit is not included in the master branch, the update script fails.